1. Field of the Invention
The invention relates to the field of integrated circuit devices. More particularly, the invention relates to circuits for sensing the state of a fuse device.
2. Background Information
In many integrated circuits, fuses are used to store information, form connections, program elements for redundancy, store identification or other information, or trim analog circuits by adjusting the resistance of a current path. These functions are typically referred to as “programming” a fuse.
To determine whether a fuse has been programmed, circuits that sense the state of fuses ususally distinguish between programmed and unprogrammed fuses by detecting a change in the resistance of the fuse device from a low to a high value. Sometimes the difference in resistance between a programmed fuse and an unprogrammed fuse is so small that the resistance difference is difficult to detect. This is especially true for fuses with smaller geometries (e.g., line widths and device sizes), whose resistances can be harder to control in the manufacturing process. Conversely, sometimes the difference in resistance between a programmed fuse and an unprogrammed fuse is so large that there is a wide range of programmed resistance values as compared to their unprogrammed resistance values. This can be the case for polysilicon fuses whose unprogrammed resistance can vary by several ohms while the programmed resistance can vary across hundreds of ohms.
To accommodate newer technologies, circuits that sense the state of fuses must be sufficiently sensitive to reliably detect small changes in resistance to accurately discern between unprogrammed and programmed fuses. Merely increasing the current in a fuse sensing circuit to increase sensitivity is not a viable approach. If the current through an unprogrammed fuse is not low enough during sensing, the unprogrammed fuse may be erroneously programmed.
Additionally, reduced supply voltages in newer technologies results in smaller signals. As a result, fuse sense circuits operating at the lower supply voltages may not have sufficient gain to ensure accurate sensing.
Other issues common to integrated circuits must also be considered when designing circuits that sense the state of fuses. For example, voltage and current characteristics of integrated circuits typically change as the ambient temperature changes. Integrated circuits also have natural mismatches among components.